In recent years, optical disc apparatuses which optically record information onto a storage medium or reproduce optically-recorded information have gained wide prevalence. As such storage media, for example, optical discs such as the compact disc (hereinafter abbreviated as “CD”), the Digital Versatile Disc (hereinafter abbreviated as “DVD”), and the Blu-ray Disc (hereinafter abbreviated as “BD”) are known. Various kinds of information, e.g., video, images, and audio, can be recorded on an optical disc.
In particular, the DVD and the BD, which are expected for use in recording video information that entails large amounts of information, e.g., movies, are facing desires to accommodate a work of long hours on a single disc, as well as intense needs to enhance the value-added of a packaged medium by storing various bonus videos. Therefore, for an increased capacity of storable information, optical discs possessing two recording layers have already been put to practical use, and are widely used on the market. In the case of the ED, which is capable of recording high-quality video information, studies are undertaken to adopt multiple recording layers for a further increase in its capacity, and discs and devices that are adapted for three layers or four layers are considered for standardization.
For example, in order to reproduce information from an optical disc, it is necessary to converge laser light onto a recording layer of interest, and detect reflected light therefrom by using a photodetector. However, as the layers of a disc increase in number, there arises a problem in that reflected light from a layer(s) other than the layer from which information is being reproduced (hereinafter referred to as “stray light from other layers”) may enter the photodetector to cause noises, thus deteriorating the qualities of the reproduction signal and control signals.
The above problem will be described with respect to a construction in which focus signal detection based on an astigmatic method using a cylindrical lens is performed, and a construction in which tracking signal detection based on a three beam method is performed, as specific constructions for a generic optical pickup device.
FIG. 28 shows an exemplary construction of a generic optical pickup. Light going out from a semiconductor laser 1 as a light source is transmitted through a diffraction grating 13 for generating three beams, reflected by a polarization beam splitter 2, and converted by a collimating lens 3 to a substantially parallel light beam. This parallel light beam is reflected by a mirror 4, transmitted through a wavelength plate 5, and converged by an objective lens 6 onto an optical disc 7 as a storage medium.
The optical disc 7 has at least three recording layers. In the present specification, the three adjoining recording layers are designated as recording layers 7a, 7b, and 7c, appearing in this order as seen from the objective lens 6. Hereinafter, the recording layers 7a, 7b, and 7c will be referred to as an L2 layer, an L1 layer, and an L0 layer, respectively.
FIG. 28 shows an optical path of light which is converged by the recording layer 7b. The reflected light from the L1 layer (recording layer 7b) reaches the polarization beam splitter 2 through an opposite path. At this point, due to the action of the wavelength plate 5, the polarization state of the reflected light has been converted to a state which is different from the polarization state in the forward path; therefore, much of the light reaching the polarization beam splitter 2 is transmitted, passes through a cylindrical lens 11, and enters a photodetector 15. The light entering the photodetector 15 will hereinafter be referred to as “detected light 9”. The detected light 9 contains three beams, i.e., a main beam 9a and sub beams 9b and 9c. 
FIG. 29 shows the construction of the photodetector 15. The main beam 9a enters a photodetecting portion 151, whereas the sub beams 9b and 9c respectively enter photodetecting portions 152 and 153. An RF signal is generated from a light amount signal of the main beam 9a as detected at the photodetecting portion 151. On the other hand, a focus error signal and a tracking error signal are generated by using, in addition to the light amount signal of the main beam 9a as detected at the photodetecting portion 151, the light amount signals of the sub beams 9b and 9c as detected by the photodetecting portion 152 and the photodetecting portion 153. The principles of detection of the RF signal, the focus error signal, and the tracking error signal are already known, and these detection principles in themselves do not pertain to the essence of the present invention; therefore, the detailed descriptions thereof are omitted.
FIG. 30 shows an optical path of reflected light from the rearward-adjoining L0 layer (recording layer 7c) when light for recording or reproduction of information is converged on the L1 layer (recording layer 7b). Since the reflected light from the L0 layer once becomes focused between the collimator lens 3 and the photodetector 15, it enters the photodetector 15 in a greatly defocused state. FIG. 31 shows reflected light 9d from the L0 layer upon the photodetector 15. The reflected light 9d is defocused, and has a large expanse on the photodetector 15, spreading over the photodetecting portions 151 to 153. Therefore, the reflected light 9d has interference with the main beam 9a and the sub beams 9b and 9c, from which the RF signal and focusing and tracking error signals are generated.
FIG. 32 shows an optical path of reflected light from the frontward-adjoining L2 layer (recording layer 7a) when light is converged on the L1 layer (recording layer 7b). The reflected light from the L2 layer does not become focused before the photodetector 15, and enters the photodetector 15 in a greatly defocused state.
FIG. 33 shows reflected light 9e from the L2 layer upon the photodetector 15. The reflected light 9e is defocused, and has a large expanse on the photodetector 15, spreading over the photodetecting portions 151 to 153. Therefore, the reflected light 9e has interference with the main beam 9a and the sub beams 9b and 9c, from which the RF signal and focusing and tracking error signals are generated.
Due to the influences of manufacturing errors and the like, the inter-layer thickness between the L1 layer and the L2 layer is not always constant, but may locally fluctuate. Therefore, the optical path length will change while the disc makes one rotation. As a result, the state of interference will always be changing. Therefore, when information is reproduced or recorded by using a multilayer disc of three layers or more with an optical pickup having such a construction, the stray light from the forward and rearward layers will interfere with the main beam 9a and the sub beams 9b and 9c, and the RF signal and focusing and tracking error signals will always be changing in amplitude and offset. This is a cause for substantial degradation of the qualities of the reproduction signal and control signals.
Regarding this problem, where the stray light from other layers affects the control signals, a solution as shown in Patent Document 1 has been proposed. FIG. 34 is a diagram for describing the construction and operation of an optical pickup device described in Patent Document 1.
In Patent Document 1, photodetecting portions for tracking error signal detection are placed at positions which are not struck by reflected light from any other layers, and a diffraction element is employed to direct a beam to be used for tracking error generation toward such positions. This makes it possible to detect a high-quality tracking error signal which is free from the influences of reflected light from the other layers, thus ensuring stability of the tracking operation on a multilayer disc. This will be specifically described below.
Light going out from a semiconductor laser 1 as a light source is reflected by a polarization beam splitter 2, and converted to a substantially parallel light beam by collimating lens 3. This parallel light beam is reflected by a mirror 4, transmitted through a wavelength plate 5, and converged by an objective lens 6 onto an optical disc 7 as a storage medium. FIG. 34 shows an optical path of light converged on the L1 layer.
The reflected light from the L1 layer reaches the polarization beam splitter 2 through an opposite path. As in the earlier example, the polarization state has been converted at this point, and therefore much of the light reaching the polarization beam splitter 2 is transmitted so as to further enter a diffraction element 8.
The detected light 9 (0th order light) which is not diffracted by the diffraction element 8 moves straight, and passes through a cylindrical lens 11 to enter a photodetector 12. On the other hand, the detected light 10 (diffracted light) which is diffracted by the diffraction element 8 strikes different positions on the photodetector 12 from that of the detected light 9.
FIG. 35 shows the construction of the photodetector 12. The detected light 9 enters the four-divided photodetecting portion 121. By using the detected light 9, RF signal detection and focus error signal detection by the astigmatic method are performed. On the other hand, the detected light 10 is split by the diffraction element 8 into four beams 10a, 10b, 10c, and 10d in a region-by-region manner, which respectively strike photodetecting portions 102a, 102b, 102c, and 102d. By using the beams 10a, 10b, 10c, and 10d composing the detected light 10, tracking error signal detection is performed. The principles of detection of the RF signal and the focus error signal (astigmatic method) pertain to an already known technique, and the principles of detection of the tracking error signal are described in detail in Patent Document 1, and these detection principles in themselves do not pertain to the essence of the present invention; therefore, the descriptions thereof are omitted.
FIG. 36 shows an optical path of reflected light from the rearward-adjoining L0 layer when light is converged on the L1 layer. The reflected light from the L0 layer once becomes focused between the collimator lens 3 and the photodetector 12, and enters the photodetector 12 in a greatly defocused state.
FIG. 37 shows reflected light 9d from the L0 layer upon the photodetector 12. Since the reflected light 9d is defocused, it protrudes widely off the photodetecting portion 121. However, the photodetecting portions 102a to 102d for tracking error signal detection are provided outside the reflected light 9d, they are not struck by the reflected light 9d. Therefore, the tracking error signal is not affected by the reflected light from the adjoining rear recording layer.
FIG. 38 shows the behavior of reflected light from the frontward-adjoining L2 layer when light is converged on the L1 layer. The light reflected by the L2 layer does not become focused before the photodetector 12, and enter the photodetector 12 in a greatly defocused state.
FIG. 39 shows reflected light 9e from the L2 layer upon the photodetector 12.
Since the reflected light 9e is defocused, it protrudes widely off the photodetecting portion 121. However, since the photodetecting portions 102a to 102d for tracking error signal detection are provided outside the reflected light 9e, they are not struck by the reflected light 9d. Therefore, the tracking error signal is not affected by the reflected light from the adjoining frontward recording layer.